The present invention is directed to a system for delivering high energy laser light by means of an optical waveguide, and in one particular application is concerned with laser angioplasty and a means for guiding such a system.
The use of laser energy to ablate atherosclerotic plaque that forms an obstruction in a blood vessel is presently being investigated as a viable alternative to coronary bypass surgery. This procedure, known as angioplasty, essentially involves insertion of a fiber optic waveguide into the vessel, and conduction of laser energy through the waveguide to direct it at the plaque once the distal end of the waveguide is positioned adjacent the obstruction. In certain uses, to enable the physician to ascertain the location of the waveguide as it is being moved through the vessel, additional waveguides for providing a source of illuminating light and for conducting the image from inside the vessel to the physician are fed together with the laser waveguide.
Most of the experimentation and testing that has been done in this area has utilized continuous wave laser energy, such as that produced by Argon Ion, Nd:YAG or Carbon Dioxide lasers. The light produced by this type of laser is at a relatively low energy level. Ablation of the obstruction is achieved with these types of lasers by heating the plaque with constant laser power over a period of time until the temperature is great enough to destroy it.
While the use of continuous wave laser energy has been found to be sufficient to ablate an obstruction, it is not without its drawbacks. Most significantly, the destruction of the lesion is uncontrolled and is accompanied by thermal injury to the vessel walls immediately adjacent the obstruction. In an effort to avoid such thermal injury and to provide better control of the tissue removal, the use of a different, higher level form of laser energy having a wavelength in the ultra-violet range (40-400 nanometers) has been suggested. See, for example, International Patent Application PCT/US84/02000, published Jun. 20, 1985. One example of a laser for producing this higher level energy is known as the Excimer laser, which employs a laser medium such as argon-chloride having a wavelength of 193 nanometers, krypton-chloride (222 nm), krypton-fluoride (248 nm), xenon-chloride (308 nm) or xenon-fluorine (351 nm). The light produced by this type of laser appears in short bursts or pulses that typically last in the range of ten to hundreds of nanoseconds and have a high peak energy level, for example as much as 200 mJ.
Although the destruction mechanism involving this form of energy is not completely understood, it has been observed that each single pulse of the Excimer laser produces an incision which destroys the target tissue without accompanying thermal injury to the surrounding area. This result has been theorized to be due to either or both of two phenomena. The delivery of the short duration, high energy pulses may vaporize the material so rapidly that heat transfer to the nonirradiated adjacent tissue is minimal. Alternatively, or in addition, ultraviolet photons absorbed in the organic material might disrupt molecular bonds to remove tissue by photochemical rather than thermal mechanisms.
While the high peak energy provided by Excimer and other pulsed lasers has been shown to provide improved results with regard to the ablation of atherosclerotic plaque, this characteristic of the energy also presents a serious practical problem. Typically, to couple a large-diameter laser beam into a smaller diameter fiber, the fiber input end is ground and polished to an optical grade flat surface. Residual impurities from the polishing compound and small scratches on the surface absorb the laser energy. These small imperfections result in localized expansion at the surface of the fiber when the laser energy is absorbed. The high-energy Excimer laser pulses contribute to high shear stresses which destroy the integrity of the fiber surface. Continued application of the laser energy causes a deep crater to be formed inside the fiber. Thus, it is not possible to deliver a laser pulse having sufficient energy to ablate tissue in vivo using a conventional system designed for continuous wave laser energy.
This problem associated with the delivery of high energy laser pulses is particularly exacerbated in the field of coronary angioplasty because of the small diameter optical fibers that must be used. For example, a coronary artery typically has an internal diameter of two millimeters or less. Accordingly, the total external diameter of the angioplasty system must be below two millimeters. If this system is composed of three separate optical fibers arranged adjacent one another, it will be appreciated that each individual fiber must be quite small in cross-sectional area.
A critical parameter with regard to the destruction of an optical fiber is the density of the energy that is presented to the end of the fiber. In order to successfully deliver the laser energy, the energy density must be maintained below the destruction threshold of the fiber. Thus, it will be appreciated that fibers having a small cross-sectional area, such as those used in angioplasty, can conduct only a limited amount of energy if the density level is maintained below the threshold value. This limited amount of energy may not be sufficient to efficiently ablate the obstructing tissue or plaque without thermal damage.
Even if the energy density is quite high, the small beam that results from the small diameter fiber may not have a sufficiently large target area that effective ablation of the lesion results. Only a small fragment of the lesion might be ablated, and thus not provide adequate relief from the blockage. A further problem with the use of a fiber optic waveguide to direct laser energy for purposes of ablating atherosclerotic plaque is that of perforation of the blood vessel. Such perforations can be caused by the waveguide itself contacting and perforating the vessel. Such perforations can also be caused by the laser beam, particularly if the waveguide is not aligned properly within the blood vessel. The perforation problems are related to the intrinsic stiffness of the glass fibers of the waveguide and poor control of laser energy, regardless of laser source or wavelength.
Also related to the stiffness of the glass fibers is the ability to control the position of the fibers radially within the blood vessels. The conventional systems employing fiber optic waveguides within a blood vessel do not provide means for controlling radial movement within the blood vessel.
One known attempt at developing an angioplasty catheter is disclosed in U.S. Pat. No. 4,747,405. The known catheter includes a center guidewire lumen, a guidewire therein, and a single optical fiber disposed at a side of the catheter for emitting laser energy. The catheter also has a blunt leading end that does not facilitate progress through a blood vessel. A particular problem that potentially results from the disclosed arrangement of the single optical fiber and guidewire is that large segments of the lesion may become loose in the blood stream and could possibly cause an emboli. As a result, the known catheter includes a dedicated channel to remove the loosened debris.
Currently "over the wire" laser catheters are guided through coronary arteries with nondeflectable guidewires. In a laser angioplasty procedure, the physician first introduces the guidewire into the blood vessel where treatment is required. The catheter is then tracked over the guidewire and the laser is activated as required to treat the vessel. Because plaque is frequently built up eccentrically within the vessel, it is frequently desirable during a procedure to change the general direction of the guidewire, and thus the path of the catheter within the vessel. With a nondeflectable guidewire, it can be difficult to change the direction of the guidewire.
Laser angioplasty systems for the treatment of nontotal occlusions include systems that have a bundle of fibers placed concentrically around a guidewire lumen, and which are covered in a protective sheath. Small diameter catheters, e.g., 1.0 to 1.7 mm O.D., have a ring of active optical fibers that extends from the center guidewire lumen to the outer protective sheath. Any occlusive material confronted by the advancing catheter will be removed by the active fiber ring.
These small diameter catheters contain fewer fibers than large diameter catheters, e.g., 1.8 mm O.D. and greater. With the increased number of optical fibers in the large diameter catheters, catheter flexibility is reduced, thus making the catheter more difficult to use. However, large arteries require a large diameter catheter.
Efforts to guide catheters by means of ultrasonic transducers are taught in U.S. Pat. Nos. 4,576,177; 4,821,731; and 4,862,893; as well as in an article titled Intraluminal Ultrasound Guidance of Transverse Laser Coronary Atherectomy by M. A. Martinelli et al.